/*
 * Copyright (c) 1998, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.lang;

import java.lang.ref.*;

/**
 * This class extends <tt>ThreadLocal</tt> to provide inheritance of values
 * from parent thread to child thread: when a child thread is created, the
 * child receives initial values for all inheritable thread-local variables
 * for which the parent has values.  Normally the child's values will be
 * identical to the parent's; however, the child's value can be made an
 * arbitrary function of the parent's by overriding the <tt>childValue</tt>
 * method in this class.
 *
 * <p>Inheritable thread-local variables are used in preference to
 * ordinary thread-local variables when the per-thread-attribute being
 * maintained in the variable (e.g., User ID, Transaction ID) must be
 * automatically transmitted to any child threads that are created.
 *
 * @author Josh Bloch and Doug Lea
 * @see ThreadLocal
 * @since 1.2
 */

public class InheritableThreadLocal<T> extends ThreadLocal<T> {

  /**
   * Computes the child's initial value for this inheritable thread-local
   * variable as a function of the parent's value at the time the child
   * thread is created.  This method is called from within the parent
   * thread before the child is started.
   * <p>
   * This method merely returns its input argument, and should be overridden
   * if a different behavior is desired.
   *
   * @param parentValue the parent thread's value
   * @return the child thread's initial value
   */
  protected T childValue(T parentValue) {
    return parentValue;
  }

  /**
   * Get the map associated with a ThreadLocal.
   *
   * @param t the current thread
   */
  ThreadLocalMap getMap(Thread t) {
    return t.inheritableThreadLocals;
  }

  /**
   * Create the map associated with a ThreadLocal.
   *
   * @param t the current thread
   * @param firstValue value for the initial entry of the table.
   */
  void createMap(Thread t, T firstValue) {
    t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
  }
}
